In the field of transceiver design, meeting the challenge of achieving wide dynamic range for transceivers is a growing concern for designers. To address this issue, variable gain control is typically employed in transceiver blocks so that high gain amplification is applied to weak signals and low gain amplification is applied to strong signals. For designs of transceivers used in high frequency operations, in which process cost is typically high, implementing variable gain control without increasing integrated circuit (IC) or chip semiconductor area is a further challenge.
The amplification of signals in transceivers also typically involves low-noise amplifiers (LNA) or cascode amplifiers. Hence, a number of variable gain control implementations involving LNAs or cascode amplifiers have been proposed.
U.S. Pat. No. 6,046,640 to Brunner proposes an LNA, referring to FIG. 1, in which alternating current (AC) and direct current (DC) through a transistor Q23 are diverted via a transistor Q22 for reducing the gain of the LNA. However, diverting the currents away from the transistor Q23 also inadvertently affects the noise figure of the LNA substantially.
U.S. Pat. No. 6,466,095 to Suzuki proposes a gain control method for cascode amplifiers, referring to FIG. 2, in which the gain of a cascode amplifier is varied by changing the transconductance gm of a transistor FET 108 through varying a drain to source voltage (VDS) applied to the transistor FET 108. Those skilled in the art can appreciate that the transistor FET 108 behaves as a load to a transistor FET 103 in the cascode amplifier. By changing the transconductance gm of the transistor FET 108 the load as applied to the transistor FET 103, however, changes and therefore may create stability problems for the transistor FET 103.
U.S. Pat. No. 6,472,936 to Jones proposes a variable gain LNA using a variable inductor method in which variable gain control of the LNA is achieved by dividing an output signal current between the output of the LNA and the supply to the LNA by using an on-chip tapped inductor. By dividing the inductance of the tapped inductor along tap points using transistors and corresponding switches, varying inductances behaving as loads at the output of the LNA are thus realised, which in turn allows for variable gain control. However, parasitic elements are introduced by the transistors which consequently affect performance characteristics of the inductances.
While Jones proposes a variable gain LNA that addresses problems attendant on Brunner and Suzuki, ie, the problems relating to adverse transconductance gm change by using a variable inductor, to those skilled in the art it is easy to appreciate that in Jones even when any transistor is switched off (high-gain mode), the parasitic element introduced by such a transistor loads the inductive transformer therefore making any high frequency design involving the variable gain LNA a problem. Such an attendant problem hence renders practical implementation of this proposal difficult.
Additionally, high frequency blocks in a transceiver using on-chip variable inductors as matching elements or loads typically suffer from performance degradation due to process variation. Hence it is also necessary to compensate the effect of process variation on the on-chip variable inductor.
In an example of an on-chip variable inductor susceptible to process variation, U.S. Pat. No. 6,437,653 to Cruz et al. proposes an apparatus for providing variable inductance using a magnetic material, in which a variable inductor is implemented for a voltage-controlled oscillator. The variable inductor consists of a primary-spiral inductor magnetically coupled to a control-spiral inductor using magnetic material implanted between the inductors. By feeding a direct current (DC) in the control-spiral inductor, thereby changing the property of the magnetic material, the inductance of the primary-spiral inductor is varied. The drawback of this proposal is that a tedious process of implanting the magnetic material, which is critical to the operation of the variable inductor, between the primary and control-spiral inductors is required. Those skilled in the art can appreciate that such a process is costly and it is typically difficult to control the implantation of such materials.
It is therefore apparent that to address the foregoing problems there is a need for an effective and efficient solution for controlling and varying gains of wide-dynamic range LNAs or cascode amplifiers using on-chip inductive elements as loads.